1. Field of the Invention
The present invention relates to radio resource allocation in a cellular system which includes a relay station (RS) using a dedicated line and a dedicated bandwidth, i.e., in a cellular system using Radio-over-Fiber (RoF). More particularly, the present invention relates to a scheduling method and apparatus for performing packet scheduling and dynamic channel allocation in order to increase the throughput of the entire system.
2. Description of the Related Art
One of several important technical factors for continuous development of wireless communication systems is the efficient management and distribution of frequency resources. To this end, research is actively being conducted on a multi-hop transmission scheme, which is developed from the conventional single-hop transmission scheme, wherein only direct transmission from one base station (BS) to a mobile station (MS) is permitted in each cell. In a system supporting the multi-hop transmission scheme, signals output from a BS are transmitted to an MS via an RS, or may be transmitted directly to the MS.
Current cellular systems may include, for example, a single-hop system, a single-hop system supporting a repeater, or a wireless multi-hop system.
The single-hop system includes a BS per cell, without a repeater, and has a construction such that a terminal or MS is directly connected to the BS without a separate relay.
The single hop system supporting repeaters includes repeaters that are additionally installed in the single-hop system in order to enhance the signal reception performance of an MS, particularly when the MS is located in a cell boundary area or a shadow area. Such a system is referred to as a repeater system.
In the repeater system, a cell includes one BS and a plurality of repeaters, and an MS transmits/receives signals at the same time to/from the BS and the repeaters. The repeater system may be classified, for example, into a cable-optical repeater structure (wired RS) and a wireless-RF repeater structure (wireless RS) according to links between the BS and the repeaters. The cable repeater has an advantage in that signal attenuation is low, but has no mobility, and the wireless repeater has a disadvantage in that all signals are amplified and broadcasted, without distinction between a signal desired by an MS and an interference signal. Also, the wireless repeater can be installed at a relatively lower cost than the cable repeater structure, but requires antenna isolation upon constructing a network.
Some of the differences between the wireless RS multi-hop system and the wireless repeater system include that while a wireless repeater amplifies and transfers not only signals received from a BS but also interference signals input from external cells, a wireless RS multi-hop system has advantages in that not only it amplifies signals desired by an MS, but also can perform scheduling and dynamic channel allocation with respect to MSs within a sub-cell formed by the wireless RS. That is, using a wireless multi-hop RS enables a BS to transmit data to even MSs located in a shadow area, to which it is difficult for the BS to directly transmit data, thereby having advantages in that the cell coverage is expanded and the cell throughput increases.
As described above, the wireless RS multi-hop system can have an enhanced transmission performance, as compared with the single-hop system or the repeater system.
Also, the wireless RS multi-hop system may be regarded as a distributed antenna system in which a BS and an RS together manage one cell. In the distributed antenna system, each BS and RS of a cellular system acts as an antenna which transmits a signal.
Moreover, one of representative inter-antenna cooperative transmission schemes in the current cellular system is a “handoff.” The handoff allows, when a specific MS moves from the communication area of a first BS to the communication area of a second BS of an adjacent cell, the specific MS to move to the communication area of the second BS without disconnection of communication through switching to a channel of the second BS. The handoff may be roughly classified into a hard handoff scheme and a soft handoff scheme.
According to the hard handoff scheme, when a specific MS is moving to the area of a new cell (i.e. an adjacent cell), the specific MS cuts off the connection with an existing cell and makes a connection with a channel of the adjacent cell when the intensity of a signal received from the existing cell is equal to or less than a threshold value.
According to the soft handoff scheme, when a specific MS is moving to the area of an adjacent cell (i.e. a new cell), the specific MS simultaneously receives signals from both BSs when the intensity of a signal received from the new cell is equal to or greater than a predetermined value, and cuts off the connection with the current cell and receives signals only from the new cell when the intensity of a signal received from the current cell is equal to or less than a predetermined value.
In an environment where a specific MS is moving from the service area of a current cell to another cell, a communication failure phenomenon sometimes occurs. In this case, it is possible to prevent such a communication failure by making the specific MS belong to the service area of the new cell. Such a handoff technology functions to prevent a communication failure phenomenon from occurring due to movement, but does not guarantee an increase in the transmission capacity in view of the entire cell.
However, advanced types of inter-antenna cooperative transmission technologies, such as a signal combining technology, a space time coding technology, etc., can be effective in decreasing an outage probability due to deterioration of signal quality in a cell boundary area, as well as being able to increase a cell transmission efficiency. That is, when an MS receives service from two or more RSs, the quality of signals is improved and the data rate increases, as compared with the case where the MS receive service from only one RS. Also, when signals are transmitted from only one RS, the signal quality is degraded in a cell boundary area because the interference signals from adjacent cells are relatively stronger, so that the MS may be incapable of performing communication itself.
In contrast, when cooperative transmission by RSs in adjacent cells is performed, communication becomes possible in a cell boundary area not only because the number of interference sources decreases, but also because the intensity of received signals increases, thereby reducing the outage probability in terms of the entire cell. However, there is a problem in that a cooperative transmission between N number of antennas necessitates radio resources N times more than those necessitated for a single transmission.
Therefore, in spite of the disadvantage that N times more radio resources are used, application of an efficient scheduling and channel allocation method upon distributing radio resources in the entire system in order to improve the transmission capacity and fairness in the entire system.
In connection with this, Korean Patent Application No. 10-2005-0131028 entitled “Method And Apparatus For Scheduling In Communication System Using Multiple Channels,” which refers to a user-request-based parallel scheduler showing excellent performance in the conventional multi-channel environments, discloses a technology of enhancing the system efficiency by allocating a channel to a user's MS having a relatively better channel state in consideration of a multi-user diversity gain and a multi-channel diversity gain. However, the disclosed method and apparatus are unsuitable to scheduling algorithms to which advanced types of technologies, such as the signal combining technology, the space time coding technology, etc., for inter-RS cooperative transmission, can be applied.
There is an urgent need in the art to develop a new type of scheduling and channel allocation method, which can efficiently reflect a two-dimensional diversity gain, as in the conventional multi-channel environments, and simultaneously can obtain even a multi-antenna diversity gain. In other words, a scheduling and allocation method that can efficiently support multi-antenna cooperative transmission.